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xv                 HYSTRICIDAE OR GROUND  PORCUPINES              499
sixth being very small. The clavicles are well developed. A
curious fact about C* villosus is that the acetabular cavity is
perforate (on both sides), or at least only closed by membrane.
In many forms of Rodents the bone is very thin in this region.
This fact perhaps lessens the significance of the perforation of the
acetabulum of &Mdna (see p. 109).
Of the allied genus CJiaetomys, also Neotropical, there is but a
single species, which inhabits Brazil. It has a nearly completely
closed orbit, a feature which differentiates it from the last animal,
and one which also shows it to be a more modified form. The
spiny covering is less pronounced than in its allies.
Fain. 8. Hystricidae.---This family is characterised by the fact
that all its members possess spines; but the tail, if at all long,
is not prehensile, and the soles of the feet are smooth and not
covered with rough tubercles, as in the Tree Porcupines of the
next family, Erethizontidae. The clavicle is less developed than
in the arboreal forms. In the organs of digestion there are
points of a family difference between the two groups of spiny
Hodents. The tongue has serrated scales arranged in transverse
rows, which are directed backwards. A gall-bladder, though not
always present, is sometimes found ; it apparently never exists
in the arboreal Porcupines and in Erethizon. The lungs show
a great tendency to subdivision, which appears to be especially
marked in the genus ^Atherura. The caecum seems also to "be
shorter in the Ground Porcupines. In Hystrix cristata the
small intestine measures 15 feet 7 inches; the caecum, 8 inches;
the large intestine, 4 feet 4 inches:---in Alherura, africana the
caecum measures 7-j- inches; the large intestine, 1 foot 10
inches. The corresponding measurements of Synetlieres villosus
were: small intestine, *7 feet 3 inches ; caecum, 1 foot 4 inches;
large intestine, 2 feet *7 inches. In J&retMzon the caecum is
2 feet 4 inches in length. These differences are too large
and too constant in a number of presumedly allied forms to be
Mr. Parsons has directed attention* also to a number of
muscular differences, such indeed as might be expected to occur
between animals of such different habits.
The genus Hystrix embraces the better-known Porcupines. It
is a genus of wide range, extending from the East Indies to Africa*
1 Proc. Zool. Soc. 1894, pp. 251, 680.